Huntington's disease (HD) is a dominant neurodegenerative disease that is caused by the expansion of a stretch of CAG triplet repeats encoding polyglutamine (polyQ) within huntingtin (htt), the protein product of the HD gene. HD is considered to be the consequence of a deleterious gain-of-function caused by the expanded polyQ stretch that is unrelated to htt's normal function. Recent work suggests that although gain-of-function may play an important role in HD pathogenesis, a corresponding loss of normal htt function also contributes to the disease process. Our long-term objective is to use genetic and biochemical approaches to understand the role of htt's normal function in HD pathogenesis, and to discover new potential therapeutic strategies for the treatment of HD based on restoring normal htt function in HD. To accomplish this objective, we propose three specific aims that are designed to help us understand how the expanded polyQ stretch can affect normal htt function, and how a version of htt that lacks its normal short stretch of polyQ (?Q-htt) is able to rescue HD phenotypes in a mouse model for HD. In Aim 1, we will test the hypothesis that ?Q-htt is able to rescue HD mouse model phenotypes by enhancing autophagic clearance of mutant htt. We will use both cell culture and mouse models to focus on two potential mechanisms that may be responsible for ?Q-htt's effects. First, ?Q-htt may mediate the enhanced recognition of mutant htt aggregates by p62/SQSTM1, a polyubiquitin binding protein that can target such aggregates for autophagic degradation. Second, ?Q-htt may affect autophagic degradation of mutant htt aggregates indirectly by enhancing retrograde transport of autophagosomes to lysosomes. To test these mechanisms, we will characterize brains and primary neurons derived from mice expressing ?Q-htt with or without 140Q-htt expression, and conditional knockout mice lacking neuronal htt expression, for p62/SQSTM1 function. In addition, the efficiency of retrograde transport will be characterized in primary neuronal cultures by measuring organelle and dynein complex movement. Recently, we have also observed that increasing the length of the mouse htt polyQ stretch from 7Q to the normal human average length of 20Q can accelerate interactions of normal and mutant htt. To test the hypothesis that an interaction between normal and mutant htt can affect HD pathogenesis, we will compare in Aim 2, behavioral and neuropathological phenotypes in mice expressing 7Q/140Q htt, and in mice expressing 20Q/140Q htt. Finally, in Aim 3, we will test the hypothesis that ?Q-htt may interact with a novel set of binding partners and/or is resistant to 140Q-htt's potential to influence the interaction of htt with its binding partners, by using mice expressing epitope-tagged htt alleles to detect differences in the repertoire of normal and ?Q-htt interacting proteins in the presence and absence of mutant htt expression.